Hash functions: Theory, attacks, and applications

Hash functions, most notably MD5 and SHA-1, initially crafted for use in a handful of cryptographic schemes with specific security requirements, have become standard fare for many developers and protocol designers who treat them as black boxes with magic properties. This strategy had paid off until 2004—both functions appeared to have withstood the test of time and intense scrutiny of cryptanalysts. Starting last year, we have seen an explosion in the number and power of attacks on the standard hash functions. In this note we discuss the extent to which the hash functions can be thought of as black boxes, review some recent attacks, and, most importantly, revisit common applications of hash functions in programming practice.